Gas dissecting along fascial planes is the specific sign; fluid and non-enhancing deep fascia are the sensitive ones. Surgery is not delayed for imaging — CT maps the extent for the debridement.
Orient first
- Necrotising fasciitis spreads along the deep fascia, often ahead of the skin signs; the patient's pain is out of proportion.
- Gas is absent in many cases (streptococcal type 2); its absence does not exclude the diagnosis.
- Cellulitis involves skin and subcutaneous fat; fasciitis thickens and fails to enhance the deep fascia, with fluid tracking between muscle groups.
Acquire the study
- CT with IV contrast of the affected region (portal venous phase), axial thin slices with coronal and sagittal reformats; soft-tissue and lung windows for gas. Do not wait for imaging if clinical suspicion is high.
The manoeuvre
- Lung window: gas locules along the fascial planes; trace their extent in cm in all three planes.
- Soft-tissue window: deep fascial thickening, fluid along the intermuscular septa, and absent fascial enhancement on the post-contrast series.
- Muscle: reduced enhancement suggests myonecrosis.
- Look for the source: a perforated viscus, perianal abscess, injection site or ulcer.
- Report the craniocaudal extent relative to fixed landmarks (joint lines, iliac crest) for the surgeon.
What confirms it
- Gas or fluid along the deep fascia with non-enhancing fascia in a septic patient.
What licenses you to exclude it
- Stranding confined to skin and subcutaneous fat with normal deep fascia supports cellulitis — but CT does not exclude early necrotising infection; surgical exploration decides.
The classic misread
- Reading gas from a recent wound, drain or injection as necrotising infection without the fascial signs.
- Reassuring because there is no gas.